With a view to applying vacuum decarburization, chemical composition adjustment, degassing and other refining treatments to a molten steel received in a refining vessel, a method is known which comprises blowing a gas from below into the molten steel received in the refining vessel under vacuum or in open air.
A conventional method for blowing the gas from below into the molten steel in the refining vessel as mentioned above comprises using a refining vessel provided with a porous plug in the bottom wall thereof, and blowing a gas through said porous plug into the molten steel in the refining vessel. FIG. 1 is a schematic sectional view illustrating the above-mentioned conventional refining vessel provided with a porous plug in the bottom wall thereof. In FIG. 1, 1 is a refining vessel, 1' is the bottom wall of the refining vessel 1, 1" is the side wall of the refining vessel 1, and, both the bottom wall 1' and the side wall 1" are formed with refractories and covered by a protecting steel sheet over the outer surface thereof.
The bottom wall 1' of the refining vessel 1 is provided with a gas blowing aperture 2 passing through the bottom wall 1'. A porous plug 3 having a shape just fitting in the gas blowing aperture 2 and made of a porous refractory is releasably inserted into the gas blowing aperture 2 from the outside of the bottom wall 1'. The porous plug 3 has such a permeability (i.e., porosity) that allows passing of a gas but not a molten steel. When a molten steel 4 is received in the refining vessel 1, therefore, the received molten steel 4 is prevented by the porous plug 3 from flowing out from the refining vessel 1. When a gas having a pressure of over the static pressure of the molten steel 4 is blown from below as shown by the arrow 5 in the drawing through the porous plug 3 into the molten steel 4 for the purpose of refining the molten steel 4 received in the refining vessel 1, the molten steel 4 in the refining vessel 1 is stirred and refined by the gas blown through the porous plug 3. Then, when gas blowing is discontinued, the molten steel 4 in the refining vessel 1 is prevented by the porous plug 3 from flowing out from the refining vessel 1.
As mentioned above, the molten steel 4 in the refining vessel 1 never flows out through the porous plug 3 before gas blowing and even after discontinuing gas blowing, by using a refining vessel 1 equipped with a porous plug 3 in the bottom wall 1' thereof. It is therefore possible also to interrupt gas blowing during refining.
However, because the permeability (i.e., porosity) of the porous plug depends upon the material particle size, the firing temperature and other manufacturing conditions, it is necessary to closely control the manufacturing conditions mentioned above when manufacturing a porous plug, thus requiring higher manufacturing costs. Furthermore, a slight change in the manufacturing conditions tends to cause variation in permeability of the porous plug manufactured. When employing a porous plug having a permeability of over a certain value, a trouble may be caused in which the received molten steel flows out through the porous plug from the refining vessel. Since the amount of gas blown through a porous plug is limited to a certain extent, it is impossible to blow a gas at a high flow rate into the molten steel. As a solution to this inconvenience, a method is known which comprises inserting a plurality of porous plugs into the bottom wall of the refining vessel and blowing a gas simultaneously through said plurality of porous plugs. This method is however problematic from the point of view of economy and safety. Furthermore, because the porous plug may easily be broken, insertion thereof into the bottom wall of the refining vessel should be carefully conducted, thus requiring a long period of time for replacing the porous plug, and hence leading to a low operational efficiency of the refining vessel.
Another conventional method for blowing a gas from below into a molten steel received in a refining vessel is known, which comprises using a refining vessel provided with a gas blowing hole in the bottom wall thereof, and blowing a gas through the gas blowing hole into the molten steel received in the refining vessel. The gas flowing hole of the refining vessel used in the above-mentioned conventional method for blowing a gas has a diameter of from about 10 mm to about 20 mm. It is therefore possible to blow the gas at a high flow rate into the molten steel in the refining vessel.
In the above-mentioned conventional method for blowing a gas, however, the molten steel in the refining vessel flows out unless, before gas blowing and after discontinuing gas blowing, the refining vessel is tilted so that the hole becomes higher in altitude than the molten steel level in the refining vessel. For this purpose, it is necessary to install a tilting mechanism of the refining vessel, thus requiring high installation costs. Furthermore, if start of gas blowing into the molten steel is not closely associated in timing with start of tilting the refining vessel, the molten steel flows out from the hole and may cause a serious accident.
Under such circumstances, there has been a demand for developing a method for blowing a gas from below into a molten steel in a refining vessel, which, before gas blowing and after discontinuing gas blowing, permits easy and certain prevention of the molten steel in the refining vessel from flowing out from the gas blowing aperture provided in the bottom wall of the vessel, allows blowing of the gas in a large quantity into the molten steel in the refining vessel, and enables to freely select a flow rate of the gas to be blown, but such a method is not as yet proposed.